Persisting DNA synthesis in SV40-transformed cells in the presence of cycloheximide

Abstract

INHIBITION of protein synthesis decreases the rate of DNA synthesis in both prokaryotes and eukaryotes. In prokaryotes, DNA synthesis is dependent on protein synthesis only for the initiation of replication¹. In the case of eukaryotes, reduction of DNA synthesis occurs rapidly after the inhibition of protein synthesis^2,3, although the mechanism of the tight coupling between protein and DNA synthesis is unclear. The DNA which is synthesised at a reduced rate in these conditions is basically normal except that the elongation step seems to be modified^2,3. Newly replicated DNA synthesised in the presence of cycloheximide seems to be associated with less chromatin protein, because chromatin synthesised in these conditions is more susceptible to digestion by DNases^3,4. T antigen of simian virus 40 (SV40) is a product of the A gene of the virus⁵ and has been shown to have a role in the establishment and maintenance of transformation^6–9. There is evidence that T antigen is required for the initiation of viral DNA replication¹⁰ and the induction of synthesis of cell DNA^11–15. It has also been suggested that T antigen initiates DNA synthesis in transformed cells and thereby serves to retain those cells in the transformed state^10,16–18. Recently, T antigen was purified almost to homogeneity, and its microinjection into quiescent mouse cells induced DNA synthesis¹⁹. However, the mechanism of action of T antigen in cell DNA synthesis remains to be clarified²⁰. If T antigen acts on cellular DNA synthesis, SV40-transformed cells which contain T antigen may behave differently from other cell lines in their ability to continue DNA synthesis in conditions of protein synthesis inhibition. We report here that DNA synthesis in SV40-transformed cells continued at an almost normal rate while protein synthesis was inhibited by cycloheximide, whereas DNA synthesis in malignant tumour cells without T antigen did not continue in these conditions.